With rise in deployment of Long Term Evolution (LTE) and LTE advanced (LTE-A), small cells using low power nodes such as Pico cells and Femto cells are considered promising to cope with mobile traffic explosion. A small cell using a low power node, which has transmission power (Tx) lower than a macro node and Base Station (BS) classes is preferred for hotspot deployments in indoor and outdoor scenarios resulting in enhanced performance.
The small cell enhancement for Evolved Universal Mobile Telecommunication System (UMTS) Terrestrial Radio Access Network (E-UTRAN) and E-UTRA focuses on additional functionalities for enhanced performance in hotspot areas for indoor and outdoor using the low power nodes.
3GPP (3rd Generation Partnership Project) is considering use of potential higher layer technologies for enhanced support of small cell deployments in E-UTRA (Evolved UMTS (Universal Mobile Telecommunications System) Terrestrial Radio Access) and E-UTRAN (Evolved UMTS Terrestrial Radio Access Network) to fulfill the deployment scenarios and the requirements specified in TR 36.932.
3GPP is considering a deployment scenario, in which different frequency bands are separately assigned to macro layer and small cell layer, respectively. Small cell enhancement is expected to support significantly increased user throughput for both downlink and uplink with main focus on typical user throughput considering a reasonable system complexity. Small cell enhancement is expected to target the capacity per unit area (e.g. bps/km2) to be as high as possible, for a given user and small cell distribution, typical traffic types and considering a reasonable system complexity. The small cell enhancements are also expected to consider the impact of the actual backhaul delays and provide solutions with the aim of improved system performance. Other aspects, for example service quality of VoLTE (Voice over LTE (Long Term Evolution)) (e.g. MOS (Mean Opinion Score)) and delay/jitter impacts on services (video streaming, video calls and so on), could also be addressed later.
In LTE Release-10 carrier aggregation, all the component carriers involved in carrier aggregation is handled at the same eNB (co-located) and the component carriers are from the same frequency band i.e. intra-band carrier aggregation. In LTE Release-11 specification supports inter-band carrier aggregation where the component carriers are from different frequency bands. In the inter-band carrier aggregation scenario, the component carrier (F1) from a lower frequency band can provide coverage and mobility whereas the other component carrier (F2) from a higher frequency band can provide high throughput to the UE. The inter-band carrier aggregation could be non-co-located, where the UE is carrier aggregated with at least one first serving frequency by a Master eNodeB (MeNB) and at least one second serving frequency served by a Secondary eNB (SeNB). When carrier aggregation between at least one cell controlled by two geographically separated eNBs is envisioned then it is called as inter-eNB carrier aggregation and the UE is said to be configured with dual connectivity mode of operation. In such a scenario, dual connectivity is envisioned such that the UE maintains physical links with at least one cell controlled by two geographically separated eNBs. The UE maintains dual connectivity both in downlink and uplink or only downlink. In uplink the dual connectivity towards the MeNB and the SeNB could be simultaneous or could be time multiplexed.
In the so-called dual connectivity mode of operation, the UE consumes radio resources provided by two different network nodes (i.e. MeNB associated with at least one first serving frequency and SeNB associated with at least one second serving frequency) connected via a non-ideal backhaul interface (X2 interface). The MeNB also called first eNB is responsible for managing the resources i.e. carriers or cells belonging to MeNB referred as Master Cell Group (MCG). The MeNB is the eNB which hosts the Radio Resource Control (RRC) layer and a single S1-MME termination point exists for an UE configured with dual connectivity mode of operation between the Mobile Management Entity (MME) and the E-UTRAN. The MeNB therefore acts as a mobility anchor towards the core network (CN). The SeNB (also called second eNB) is an eNB providing additional radio resources for the UE. The SeNB is responsible for managing the resources i.e. carriers or cells belonging to SeNB referred as Secondary Cell Group (SCG). The eNB configured as SeNB for a given UE could also be operated as a normal LTE cell for standalone UEs. The E-UTRAN architecture and related functions to support Dual Connectivity for E-UTRAN is further described in TS 36.300.
The standard procedure for the intra-frequency and inter-frequency LTE (i.e. handover between LTE cells) handover in LTE specification (i.e. 3GPP TS 36.331) is described herein.
An event A1 is triggered when the signal strength of the serving cell (wherein the UE is connected to the serving cell), which is measured by the UE, greater than a threshold value.
An event A2 is triggered when the signal strength of the serving cell, which is measured by the UE, is less than a threshold value, wherein the UE is connected to the serving cell, but the signal strength of the serving cell is not strong enough.
An event A3 is triggered when the signal strength of a neighboring cell (managed by the same or a different eNB) is greater than that of the serving cell and the difference between the signal strengths of the serving cell and the neighbor cell is greater than a predefined offset. If the event A3 is detected, the UE predicts that a handover will occur in the near future. Event A3 is applicable for both intra-frequency and inter-frequency LTE cells for relative comparison.
An event A4 is triggered when the signal strength of the neighboring cell (managed by same or a different eNB), which is measured by the UE, is greater than a threshold value.
An event A5 is triggered when the signal strength of the serving cell is less than a predefined first threshold value and the signal strength of a neighboring cell (managed by same or a different eNB) is greater than a predefined second threshold value. Here, the first threshold value can be less than the second threshold value. Event A5 triggering is expected when the UE is moving out of the source cell and entering a neighboring cell. If the event A5 is detected, the neighboring cell (managed by same or a different eNB) becomes the target eNB for a handover such that the User Equipment (UE) detaches from the serving cell. Event A5 is applicable for both intra-frequency and inter-frequency cells for relative comparison.
Furthermore, in multi-carrier LTE i.e. LTE operation in carrier aggregation (CA) mode specified in LTE Release-10 specification, a new measurement event have been defined as Event A6 (intra-frequency neighbor becomes offset better than Serving Cell, on secondary carrier). With this event, it is possible to evaluate the relative strength of cells on a carrier, where the concerned carrier does not have to be the primary carrier managed by the primary serving cell, or PCell. With this event, the network can make a cell change on the secondary LTE carrier.
According to 3GPP specification TS 36.300 for control plane (C-plane) architecture in dual connectivity there is no termination for the RRC (Radio Resource Control) protocol in the SeNB towards the UE. In inter-eNB carrier aggregation employing dual connectivity, the configuration of cell(s) (carriers) that belongs to the SeNB can be provided by the MeNB depending on data demand from the UE and the signal strength of the SCG cells. The SCG cells are secondary cells (SCells) similar to SCells in Rel-10 CA. The UE needs to do synchronization towards the SCG cells since the SeNB is geographically separated. Since the RRC layer is only sitting in the MeNB, the MeNB is responsible for radio resource management and associated measurement configuration both for the MCG cells and SCG cells.
Similar to Rel-10 CA, for dual connectivity mode of operation of the UE, the primary cell (PCell) of the UE belongs to the MCG. UE can be configured with additional SCell(s) served by a set of frequencies in the MCG. The set of frequencies handled by the MeNB are referred to as a first set of frequencies. There can be at least one serving cell for the UE associated with a first serving frequency served by the MeNB. Within the SCG, the UE does not have PCell, but a cell having similar functionality like the PCell is configured to the UE. This special SCell within the SCG is called special SCell (pSCell or PSCell) where PUCCH resources are configured. The special SCell or PSCell in the SCG configured for the UE is used for transmission of HARQ feedback for PDSCH scheduled on SCG SCells and also the CQI reports for concerned SCG SCells over the configured PUCCH resources. UE can be configured with additional SCell(s) in the SCG served by a set of frequencies different from the first set of frequencies. The set of frequencies handled by the SeNB are called second set of frequencies. There is at least one serving cell for the UE associated with a second serving frequency served by the SeNB. Furthermore, the SCells within the MCG and SCG do not have PUCCH (Physical Uplink Control Channel) configuration but can provide additional PUSCH resources to the UE. The configuration of pSCell/PSCell for the UE within the SCG can be any of the configured SCells within the SCG. Assuming that SeNB is handling more than one SCell and each SCell on a different frequency, the Event A6 cannot be used for special SCell (pSCell/PSCell) change configured for the UE since Event A6 is only applicable for intra-frequency measurements. Further, Events A2 and Event A4 cannot be used because they cannot be used for relative comparison of signal strength, they can be used only for addition/removal of the carrier.
Furthermore, Events A3 and A5 cannot be used because currently these events are applicable only for the PCell. Hence in the above mentioned scenario for a UE configured with dual connectivity mode of operation, handling of special SCell (pSCell/PSCell) change cannot be done using any of the above described events.
Therefore, for inter-eNB carrier aggregation i.e. dual connectivity mode of operation, the extension of measurement events A3 and A5 or new measurement event for relative inter-frequency comparison requires new RRC signaling between the UE and the MeNB.
When a plurality of Secondary Cells (SCell) are added simultaneously by the MeNB during SCG establishment, there is a need to decide/determine specific parameters of which SCell (i.e. a reference cell) are used for deriving a SeNB key by the MeNB. The indication of the reference SCell decision/determination needs to be conveyed to the UE by the MeNB to enable the UE to derive the same SeNB key. A suitable entity should be configured to decide the reference cell namely the special SCell (pSCell) whose cell-specific parameters such as physical cell identity (PCI) and/or downlink frequency (DL-EARFCN) are to be used for KeNB key derivation. The special SCell (pSCell or PSCell) seems a good candidate to be used as reference cell since the PSCell has some of the properties of primary cell (PCell) defined in 3GPP specification TS 36.300 and TS 36.331.
The identification/determination or selection of pSCell/PSCell can affect the SCG establishment and handover (HO) procedures involving the MeNB. The selection of the pSCell/PSCell is an issue that can require some coordination between MeNB and SeNB when making decision, which SCell within the SCG can be the pSCell/PSCell. The RRM measurements from the UE indicate the relative radio link quality of the plurality of SCells, which can be added during SCG establishment. The RRM measurements can be received from the UE through measurement report or some feedback from the UE. In general it would be preferable for the SeNB to control the SCG configuration as much as possible; it can be good to consider how this objective could be realized.
The SeNB can decide the special SCell (pSCell/PSCell) from the plurality of SCells added by the MeNB during SCG establishment. The SeNB can randomly decide the pSCell/PSCell from the plurality of SCells added or the SeNB can decide the pSCell/PSCell based on the physical resource availability i.e. taking the load information into account in the plurality of SCells.
If it is also required that the SeNB identified/determined or selected pSCell/PSCell has to meet an additional radio link quality criterion, then the MeNB needs to be involved, as MeNB handles the RRIVI measurements of the UE. The MeNB could assist the SeNB with measurement information i.e. by providing the SeNB with a list of SCG cells meeting the pSCell/PSCell measurement criterion (i.e. the set of SCells from which the SeNB can make a selection of pSCell/PSCell based on resource availability).
Therefore, for inter-eNB carrier aggregation i.e. dual connectivity mode of operation, the assistance procedures by which one eNB provides either RRM measurement results or a list of candidate pSCell/PSCell, as discussed above, the information exchange is considered with new signaling support on the X2 interface between the MeNB and SeNB.
Apart from this the SON (Self-Optimizing Network) related reports sent by the UE to network (eNB) are pertaining to the procedural reports and failure reports of the PCell (MeNB) and not the pScell/PSCell (special cell of SeNB). There is a need to enhance the existing reports for SON, which will provide information to SeNB through MeNB. The information exchange is considered with new signaling support on the X2 interface between the MeNB and SeNB.
In addition to this UE also need to provide UE capability to SeNB and also there is need to define new Feature Group indicator (FGI) for SeNB. The UE can provide this information to the MeNB, which can be further shared to the SeNB through the X2 interface.
The above information is presented as background information only to help the reader to understand the present invention. Applicants have made no determination and make no assertion as to whether any of the above might be applicable as Prior Art with regard to the present application.